A thin-film photoelectric conversion device having a silicon thin film etc. as a photoelectric conversion layer includes a thin-film photoelectric conversion unit having on a transparent electroconductive film a p-type semiconductor layer, an i-type semiconductor layer (photoelectric conversion layer) and an n-type semiconductor layer in this order.
Generally, a transparent electroconductive film having an irregular structure on a surface thereof is used for enhancing the efficiency of the utilization of incident light by light trapping. As a material of the transparent electroconductive film, a conductive oxide such as tin oxide is widely used. In recent years, zinc oxide has also come into use as a transparent electroconductive film of a thin-film photoelectric conversion device. Zinc oxide has a high transmittance to light in a long wavelength range, making it easy to control a haze ratio as an index of the light trapping effect, and is excellent in anti-reduction properties to hydrogen radicals. A method has been proposed in which a substrate with a pyramid-shaped or inverted pyramid-shaped underlying layer formed on a transparent substrate is provided, and a zinc oxide transparent electroconductive film is formed thereon to utilize a larger amount of incident light (see, for example, Patent Document 1).
Preferably, conductivity-type layers (p-type semiconductor layer and n-type semiconductor layer) of the thin-film photoelectric conversion unit have a high electroconductivity. Preferably, the p-type semiconductor layer has small light absorption because it is a layer disposed on the light incident side of an i-type semiconductor layer. For the p-type semiconductor layer, a p-type amorphous silicon carbide or the like is used.
It is known that the use of zinc oxide as a transparent electroconductive film causes an increase in contact resistance at the interface between a transparent electroconductive film and a p-type semiconductor layer, and a deterioration of ohmic characteristics, leading to a decrease in open circuit voltage (Voc) and fill factor (FF), and particularly, when amorphous silicon carbide is used as a p-type semiconductor layer of a photoelectric conversion unit, this tendency is noticeable. For solving the above-mentioned problem, an attempt has been made to form a contact layer between a zinc oxide layer and a p-type semiconductor layer of a photoelectric conversion unit in order to reduce the contact resistance between both layers.
For example, it is known that, when a p-type crystalline silicon layer as a contact layer is provided between a p-type semiconductor layer and a transparent electroconductive film, the contact resistance at the junction interface between the zinc oxide transparent electroconductive film and the p-type semiconductor layer is reduced. However, since p-type silicon in the contact layer is hard to be crystallized, it is difficult to sufficiently reduce the contact resistance.
Patent Document 2 discloses a method in which, as a contact layer, a first p-type crystalline semiconductor layer having a small impurity amount (dope amount) is formed, film formation is then suspended, and a second p-type crystalline semiconductor layer having a large impurity amount is formed on the first p-type crystalline semiconductor layer. According to this method, the degree of crystallization of the second p-type crystalline semiconductor layer is improved, so that a proper interface junction can be formed between a transparent electroconductive film and a thin-film photoelectric conversion unit.
It is known that the use of zinc oxide as a transparent electroconductive film causes zinc atoms to be easily diffused into a semiconductor layer (silicon layer), so that zinc forms a defect level as an impurity in the semiconductor layer, leading to a reduction in power generation efficiency. Patent Document 3 describes that a layer containing a silicon oxide is formed between a transparent electroconductive film (zinc oxide layer) and a semiconductor layer to suppress the diffusion of zinc atoms into the semiconductor layer.